The present invention relates to the composition and formulation of a photoresist material for microimaging, characterized by a novel photosensitizer which performs better in enhancing the resolution of photoresist than commercially available ones presently. Essentially, micro-imaging process comprises coating a thin film of a photoresist containing a resin matrix and a photosensitizer onto a substrate; imagewise exposing through a photomask the film to initiate photochemical reaction in the film, and developing the microimage by dissolving and washing away the portion where photoresist either has or has not undergone photochemical reaction, as the case may be. A photoresist is of positive type if it can be washed away by a developer where it has been exposed to light. It is of negative type if it can be washed where it has not been exposed to light.
Due to the higher resolution and better thermostability positive photoresist compositions can offer, they are becoming the predominant of the above two types of photoresists in the manufacturing of miniaturized electronic components and micro-image production. The two major components of positive photoresist compositions are a resin matrix and a photosensitizer which are blended to form the compositions. The resin matrix is generally a copolymer of cresol and formaldehyde, and is called novolak, and is soluble in common organic solvents and alkaline solutions. Photosensitizers used for positive photoresist are naphthoquinone diazide derivatives. The coated film made from novolak and 15 to 30% by weight of such a photosensitizer dissolves much more slowly in an alkaline solution than films of novolak alone. However, if these photosensitizers are exposed to actinic radiation, the photochemical reaction transforms it into a carboxylic product which is more soluble in alkaline solution. The reaction is illustrated as follows; ##STR1##
The photochemical mechanism for the sensitizer was proposed by Sus (1944). Upon reacting with light, the quinone diazides form ketene intermediate and converts to carboxylic acid group which is base soluble. In unexposed area, a chemical reaction called "coupling" occurs instantly in alkaline developing solutions, which also remove the exposed residues. This photochemically generated difference in dissolution rate in alkaline developing solution is made use of by the production of micro-images. The chemistry of photoresist material and the production of micro-images have been described in the book "Photoresist Materials and Processes," by De Forest, W. S., McGraw Hill Book Company, N.Y., 1975, which is incorporated herein by reference.
U.S. Pat. Nos. 3,046,118, 3,148,983, 3, 3,402,044, 4,115,128, 4,173,470, 4,550,069, 4,551,409, Japan Patent Nos. 60-134235, 60-138544, 60-143355, 60-154248, 60-164740, 60-176034 and European Patent Application No. 0,092,444 disclose naphthoquinone diazide photosensitizers, such as 1-oxo-2-naphthoquinone-5-sulfonic acid chloride, 1-oxo-2-naphthoquinone-4-sulfonic acid chloride, and the esterified condensate of trihydroxybenzophenone for micro-image production.
One drawback of conventional photoresist compositions containing novolak resins and naphthoquinone diazide photosensitizer useful in the near UV spectral region 360-450nm) is that it shows low sensitivity when utilized in mid-uv region (300-350nm). This adversely affects wafer throughput. Due to low optical absorption or incomplete bleachability of the sensitizer, the photochemical product possesses inefficient transparency. Therefore photo reaction cannot be effected at the bottom layer of the resist and well-defined patterns can hardly be obtained.
FIG. 1 shows the absorption spectrum for AZ1350 J photoresist, available from American Hoechst Corporation, before and after bleaching. It indicates that there is no photobleaching effect after exposing at wavelength 313 nm which is the major component in the mid UV region, and as a result it is not suitable for exposure in mid-UV light.
In view of the fast development of higher circuitry density in miniaturized electronic devices, in order to increase the resolution in forming microimages, i.e. to reduce the line width, the most important method is to expose the photoresist by decreasing the wavelength of the incident irradiation. For example, exposures with mid-UV radiation source of wavelength 313 nm can achieve this object. A reduction in the exposure wavelength from near UV to mid-UV at 313 nm would substantially increase the resolution.
Due to the lack of sensitivity for conventional photoresist in the mid UV region, especially at 313 nm, the employing of a shorter wavelength radiation so as to increase image resolution cannot be achieved. Currently, mid-UV photoresists have already been developed, such as 2,3,4-trihydroxybenzophenone and 1-oxo-2-napthoquinone-4-sulfonic acid triester which are disclosed in U.S. Pat. No. 4,596,763 and 4-benzyl-1,2,3-trihydroxybenzene and 1-oxo-2-diazo-naphthalene-5-sulfonic acid ester which is disclosed in U.S. Pat. No. 4,737,437.